Infection remains the most frequent illness of man and one of the most common causes of death. The investigations proposed here seek to gain understanding of the physiology of host defense against infections, especially bacterial and fungal infection. The ultimate goals of this research are the more effective prevention and management of infections in the normal and compromised host and the eventual manipulation of certain functions of the immune system to enhance its protective effects. The focus of these studies is the oxygen-dependent mechanisms by which phagocytic cells (neutrophils and macrophages) kill microorganisms. In particular, the projects proposed here seek to gain knowledge about the molecular nature of the events that comprise the "respiratory burst system": binding of the stimulus (organism) to the phagocyte membrane ->initiation and transduction of a bio-chemical signal -> triggering of the enzyme responsible for converting oxygen to its microbicidal metabolites. Most of the projects explore elements of the process of signal transduction in phagocytes. The biochemical events under scrutiny will be compared in normal and activated macrophages, which are capable of markedly increased activity of the respiratory burst, and in neutrophils "primed" for such activity by exposure in vitro to a variety of agents, especially endotoxin and cytokines. The intent is to gain understanding of the molecular basis for upregulation of the respiratory burst and of the mechanisms responsible for triggering the respiratory burst enzyme. Specific projects will compare different priming agents and relate changes induced in the time course of the respiratory burst to changes in intracellular free Ca2+ (at baseline and stimulated, in individual cells with laser cytometry and in cell populations), in five major products of membrane phosphoinositide remodelling, and in activities of phospholipases C and A2. The molecular basis of the priming induced by brief exposure of macrophages and neutrophils to proteases will be sought by similar analyses. The effect of protein malnutrition on the activation of macrophages will be studied in mice. Improved understanding of the mechanisms responsible for the respiratory burst of phagocytic cells might permit its eventual pharmacologic modulation.